Venus death dive to unlock secrets of Earth's evil twin

On its final plunge, the Venus Express probe will fly deeper than ever before – but a return mission could tell us more about climate change and alien life

TIME to wave goodbye to Earth's fiery twin. Next week, a European spacecraft will start a series of dives into the hellish atmosphere of Venus, marking the beginning of the end for the only probe now orbiting the planet.

The dives will take the craft, called Venus Express, deeper into the atmosphere than it has gone before, allowing it to record conditions in a largely unstudied region. It will also be a test of the spacecraft's endurance as it drags itself through the planet's thick air, which will provide valuable data for future interplanetary missions.

Venus Express may not survive the month-long campaign of daredevil plunges. Even if it does, the craft will run out of fuel later this year. No dedicated probe is due to launch in the next decade, and a damaged Japanese craft has just a slim chance of making it there next year.

But there is still so much to discover about our neglected neighbour. Better knowledge of Venus could help answer two of the most important questions in modern science: how is Earth's climate changing, and are we alone in the universe?

"Venus is so similar to Earth and yet so different," says project scientist Håkan Svedhem at the European Space Agency (ESA). "One really needs to understand Venus to understand all terrestrial planets."

At first glance, Venus seems nothing like Earth. It is shrouded in a haze of carbon dioxide, with toxic sulphuric clouds and temperatures topping 450 °C. Its surface is bone dry, and the air pressure is high enough to rupture the hull of a submarine.

However, Venus is almost exactly the same size and mass as Earth, and is made from similar materials. It is thought to have started out with a water-rich atmosphere like Earth's, which may even have made the surface briefly habitable. But Venus is closer to the sun and lacks a global magnetic field, which is what protects Earth from our star's harshest rays.

Without this shield, young Venus was blasted by radiation that boiled away most of the water in its air, leaving dense carbon dioxide and triggering a runaway greenhouse effect.

"Venus is like a controlled experiment: what would happen if you took another Earth and started it off in slightly different conditions," says David Grinspoon, a NASA astrobiologist who worked on Venus Express.

During its eight years in orbit, the Venus Express mission has made many discoveries about our "evil twin", most notably about the planet's wind patterns.

"It has really been our first weather satellite on another planet," says Grinspoon. "A lot of advantages have come from observing it over a long period of time and seeing the patterns of changes in the atmosphere."

Oddly, feeding atmospheric data from Venus Express into a variety of climate models throws up some surprising results. While these simulations can faithfully reproduce conditions on Earth, they all fail to recreate the climate of Venus, says Svedhem. Figuring out why could tell us about the underlying processes of climate change, and perhaps improve our models of climate on Earth.

Such knowledge would also be useful to astronomers hunting for Earth-like worlds elsewhere in the galaxy. At the moment, we are able to identify planets with a similar mass and radius to Earth that orbit their stars at the right distance to support liquid water.

Technically, Venus falls into this category, so worlds that look friendly from afar may be hellish up close. "If we can't figure out Venus, we have no chance of predicting conditions on exoplanets," says Grinspoon.

Daring dives

For its swansong, Venus Express will perform aerobraking, a way to reduce the speed of a spacecraft and so reduce its altitude. This lets an orbiting probe get much closer to a planet's surface and study it in greater detail.

On 18 June, Venus Express will take its first dip into the clouds, where it will record the effects of friction from the atmosphere. During a series of these dives until 11 July it will also take readings on the atmosphere's density and composition.

ESA has never attempted aerobraking before, so lessons from the Venus experiment will be valuable for future probes. For instance, the agency plans to use the technique when the ExoMars satellite arrives at the Red Planet in 2017, says ESA's Olivier Witasse.

Learning more about aerobraking could also be useful for human missions. A related manoeuvre, called aerocapture, would help spacecraft land on Mars or return to Earth without needing prohibitively large landing rockets.

"People think the hard part of space travel is going as fast as you can," says Grinspoon. "There is also the hard part of slowing down when you get places, because you need a big rocket for that or you just keep going."

If Venus Express survives the ordeal, the plan is to take the probe back into a higher orbit and continue observations until its fuel runs out.

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